Education in OECD’s PIAAC Study: How Well do Different Harmonized Measures Predict Skills?

The comparable measurement of educational attainment is a challenge for all comparative surveys and cross-national data analyses. While education is an important predictor or control variable in many research contexts, it is particularly important when studying education and education-related outcomes such as skills or labor market chances. This study evaluates the cross-nationally comparable measurement of education in OECD’s Programme for the International Assessment of Adult Competencies, PIAAC, in terms of its construct validity when predicting general basic skills. In order to do so, the predictive power of country-specific (i.e. non-comparable) education variables is compared to the predictive power of different cross-nationally harmonized variables, namely the detailed ISCED-based coding scheme used in PIAAC, ISCED 2011 and 1997 levels, the broad education levels ‘low, medium, high’, ES-ISCED, as well as years of education. The analyses consist in sets of country-wise linear regressions, taking PIAAC’s plausible values and complex sampling into account, and use adjusted R2 as the indicator for predictive power and validity. The results show that while harmonization into a detailed coding scheme such as the most detailed comparable variable available in PIAAC does not entail large losses of information, the way this variable is further simplified plays a major role for validity. The paper also highlights shortcomings of the detailed variable from a theoretical point of view, such as the lack of differentiation of vocational and general education and other markers of educational content and quality, which are important aspects both for skill development as well as the labor market outcomes of education, and of the country-specific measures of education, which may make the detailed PIAAC education variable look better than it actually is.